Magnetic (or magneto-resistive) random access memory (MRAM) is a non-volatile memory technology that shows considerable promise for long-term data storage. Performing read and write operations on MRAM devices is much faster than performing read and write operations on conventional memory devices such as DRAM and Flash and order of magnitude faster than long-term storage device such as hard drives.
A typical MRAM array is made up of memory cells, each of which includes a magnetic tunnel junction (MTJ) including two ferromagnetic layers separated by a non-magnetic layer (e.g., tunnel barrier layer). These magnetic layers are commonly referred to as a “fixed” or “reference” layer, in which the direction of magnetization is fixed, and a “free” layer, in which the direction of magnetization may be switched.
The resistance of an MTJ varies based on the relative directions of magnetization of these layers. For example, when the directions of magnetization of the fixed and free layers are parallel, the resistance may be relatively small (typically representing a logical “0”), and may become greater when the directions of magnetization are anti-parallel (typically representing a logical “1”).
In one conventional type of MRAM, to switch the direction of magnetization of the free layer of a particular cell, currents are applied through a bit line and a word line that intersect at that cell. The combined magnitude of the currents through the word and bit lines generates a magnetic field at their intersection that is strong enough to switch the direction of magnetization of the free layer of the selected cell.
In another conventional type of MRAM, known as “spin torque” MRAM, instead of applying magnetic fields via the bit and word lines, the direction of magnetization of the free layer is switched by passing a spin-polarized current through the MTJ of the selected cell.
In order to construct high density magnetic memories, it is desirable for the cell size to be small. Because spin torque MRAM uses write currents that are typically lower than those used for generating a magnetic field sufficient for writing, smaller cell size can be achieved. This makes spin torque switching well suited for use in high density MRAM devices.
The resistance change ΔR=RAP−RP, that is the difference between the anti-parallel (RAP) and parallel (RP or R) resistance values, divided by the parallel resistance RP is known as the magnetoresistance (MR) ratio of the magnetic tunnel junction (MTJ) and is defined as(RAP−RP)/RP=ΔR/RP=ΔR/R. 
For MTJ device applications it is important to have high signal-to-noise ratio (SNR), the magnitude of the SNR being directly proportional to the magnetoresistance ratio (MR ratio=ΔR/R) of the magnetic tunnel junction (MTJ). The signal-to-noise ratio is given by iBΔR, iB being the bias current passing through the MTJ device. However, the noise obtained by the MTJ device is determined, in large part, by the resistance R of the device. Thus, the maximum SNR for constant power used to sense the device can be obtained if the magnetoresistance (MR) ratio is large.
The resistance R of an MTJ device is largely determined by the resistance of the insulating tunnel barrier layer. Moreover, since the read and the write currents passes perpendicularly through the ferromagnetic layers and the tunnel layer, the resistance R of an MTJ device increases inversely with the area A of the device, therefore it is convenient to characterize the resistance of the MTJ device by the product of the resistance R times the area A (RA).
In order to scale to high memory capacities, MRAM cells will need to be shrunk in size, requiring low RA values so that the resistance R of the cell is not too high and sufficiently high heating or spin current densities can be used at acceptable values of reliability of the MTJ device.
Conventionally, the materials used for the insulating tunnel barrier layers are (Magnesium Oxide) MgO or Aluminium Oxide (Al2O3). For MgO or Al2O3 insulating tunnel barriers it has been found that RA increases exponentially with the thickness of the layer. The thickness of the MgO or Al2O3 insulating tunnel barrier layers can be varied over a sufficient range to vary RA by more than eight orders of magnitude, i.e. from more than 2×109Ω(μm)2 to as little as 1Ω(μm)2. For typical MgO based insulating tunnel barriers a RA product of 1Ω(μm)2 to 10Ω(μm)2 is required to withstand current densities in the order of 0.1 MA/(cm)2 to 10 MA/(cm)2. However, for these low RA values, the magnetoresistance (MR) ratio, and therefore the SNR, is typically reduced, in part because of microscopic pin holes or other defects in the ultra thin tunnel barrier layers needed to obtain these very low RA values. Moreover, the ultra thin tunnel barrier layers needed to obtain these very low RA values reduces the barrier reliability.
Therefore there is a need in the art for MTJ devices characterized by a large tunneling magnetoresistance (MR) ratio, a reliable tunnel barrier layer and a low RA value.